Time-Symmetry
(coming soon) Time-symmetry refers to .. (see also Yakir Aharanov) Articles Taming the quantum spooks: can retrocausality solve the puzzle of action at a distance - Huw Price, Ken Wharton, aeon.co (see also Retrocausality) "Einstein’s 1905 theory of special relativity raised a new difficulty for Newton’s theory of gravity. Instantaneous action-at-a-distance requires that the distant effect is simultaneous with the local cause. According to special relativity, however, simultaneity is relative to the observer. Different observers disagree about which pairs of events are simultaneous, and there’s simply no fact of the matter about who is right. Without simultaneity at a distance, the notion of instantaneous action-at-a-distance doesn’t make sense. By making Newton’s theory of gravity even more problematic, special relativity gave Einstein an extra motivation for developing his own theory. He succeeded in his theory of general relativity (GR) 10 years later. GR explains gravity in terms of the curvature of spacetime, and abandons the idea that it acts instantaneously. In GR, gravitational effects propagate at the speed of light. If the Sun suddenly vanished, it would be eight minutes before the Earth reacted." The really bad news for Einstein came not from Copenhagen but from Belfast, from the ingenious brain of John Stewart Bell. Bell was no fan of the Copenhagen view – he saw the appeal and power of the EPR argument – but by pressing further on the same kind of two-particle experiments, he derived what seemed an insuperable difficulty for Einstein. Einstein’s argument took for granted that there is no action-at-a-distance, but Bell’s Theorem (1965) seemed to show that quantum theory requires it.". "The core of Bell’s argument can be explained using analogies. Consider what we’ll call the ‘Gemini game’. Pairs of twins are separated, and each is randomly offered one of three coloured cards: red, yellow or blue. Each twin has to accept or decline the card. If they choose differently when offered the same coloured card, they are immediately disqualified. Otherwise, their objective is just to choose differently when offered different coloured cards, as often as possible. The twins don’t know in advance what cards each of them will be offered, nor what card the other is being offered, in any particular instance. So, to avoid disqualification, they need a policy – eg, ‘Accept red, decline yellow and blue.’ Because there are three cards and only two options (accept or decline), any such policy recommends the same action for at least two different cards – in this case, for yellow and for blue. There are six ways in which the twins can be shown different cards (three possibilities for Twin 1, and for each of those, two different possibilities for Twin 2). Because any policy recommends the same option for at least two cards, it will tell the twins to do the same thing in at least two of these six situations; in our example, when Twin 1 gets yellow and Twin 2 gets blue, and vice versa. This means that the best the twins can do in their attempt to choose different options when shown different cards is four out of six, or about 67 per cent, on average. This result is what’s now known as Bell’s Inequality. Bell’s insight was to notice that somehow the quantum world manages to escape this inequality. Quantum particles can do something that even the most intelligent human twins cannot. Playing an equivalent game, for example, photons can get a success rate of 75 per cent. In the Gemini game, we assumed that neither twin knows what colour card the other is offered. Bell reasoned that for quantum particles to do better – to violate Bell’s Inequality – the equivalent assumption must fail in quantum mechanics. In some sense, each particle must ‘know’ what measurement is being made on the other. That ‘knowledge’ is the action-at-a-distance." Category:Time-Symmetry Category:Time Category:Quantum Philosophy